1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system and a communication device thereof, and more particularly, to a method of handling handover for a network of a wireless communication system and a communication device thereof.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
A term “dual connectivity” refers to an operation where a given UE consumes radio resources provided by at least two different network points connected with a non-ideal or ideal backhaul (e.g. X2 interface). Furthermore, each eNB involved in the dual connectivity for a UE may assume different roles. These roles do not necessarily depend on the eNB's power class and can vary among UEs. In dual connectivity, (i.e., an inter-node radio resource aggregation, inter-eNB carrier aggregation or inter-eNB radio resource aggregation), the UE receives a plurality of data blocks from a master eNB (hereinafter MeNB) on at least one first component carrier (CC) and from a secondary eNB (hereinafter SeNB) on at least one second CC, and transmits a plurality of data blocks to the MeNB on the at least one first CC and/or to the SeNB on the at least one second CC.
A UE in dual connectivity may be involved in a handover. For example, the UE is simultaneously served by a MeNB and a SeNB in a dual connectivity mode. When receiving a measurement report from the UE, the MeNB may make a decision to hand over the UE to a target MeNB. During the handover procedure, the UE deactivates all cells of the MeNB. However, the SeNB may continue transmitting data blocks to the UE or receiving data blocks from the UE since the SeNB does not know the MeNB is initiating the handover for the UE. During handover, the UE derives a new key for data transmission and reception. However, the data blocks transmitted by the SeNB may be ciphered with an original key. Thus, the UE uses the new key to decipher the data blocks ciphered by the original key, and will fail to correctly decipher the data blocks. Thus, there is a need for improvement over the prior art.